Process for verifying the energy of an ion beam

ABSTRACT

Process for adjusting the precise energy of an ion beam, generally less than 2 eV, emitted by an ion source into a reaction chamber, and causing ion/molecule reactions therein. The reaction chamber is filled with a calibration gas and the voltage which accelerates the ions between the ion source and reaction chamber is varied until the energy-dependent ratio between the rates of production of the two reaction products attains a pre-set value or until the energy-dependent production rate of a reaction product, the energy-dependence of said production rate showing an extreme value, reaches said extreme value.

BACKGROUND OF THE INVENTION

The invention relates to a process for verifying and adjusting theprecise value of the energy, generally being less than 2 eV, of the ionsemitted by an ion source and entering a reaction chamber as a beam,which cause ion molecule reactions in the reaction chamber, the reactionproducts of which being detectable mass spectroscopically.

On studying ion molecule reactions, ions (e.g. Kr⁺, He⁺) of definedenergy are directed to neutral molecules. The interaction of ions andmolecules can then consist in a mere charge exchange. But it is alsopossible that it comes to a chemical change of the hit molecule, e.g. bydissociation. Mass and charge of the reaction products can be analysedin a mass spectrometer. This is done today primarily to determine thecross sections of different ion molecule reactions at differentenergies.

The application of the process described in the analysis of thecomposition of the gas bombarded with ions, especially to themeasurement of air pollutants, is only possible under very certainconditions. To be able to test the exhaust gases of combustion processesfor CO, NO and NO₂ with a single ion beam, the use of an ion source isimportant, that emits ions with an energy of less than 2 eV, which onthe one hand ionize the relevant contaminations, but on the other handdo not dissociate the components of the gas composition. It has to betaken care that the ion molecule reaction takes place in the vacuumunder single collision conditions. Hence, the generated product ionsshould, in their turn, not produce further products in a mannerdifficult to control.

Also in those cases, where the ion molecule reactions of the kinddescribed are only used to measure air pollutants and details of thereaction are furthermore not interesting, it is nevertheless necessary,to hold the energy of the ions impinging on the gas to be tested at apredetermined value, as the production rate of the product ions isdifferently energy dependent. A direct measurement of the voltagebetween the emission filament of the ion source and the reaction chambercan therefore only be used for a coarse adjusting of the ion energy, asalready small changes of the surface coatings (as they occur especiallyin the presence of different gas contaminations) in the aperture systemsin the ion source or also in the reaction chamber lead to changes of thecontact potentials in the order of 0.1 Volts, whereby the energy of theions is changed by just the same amount of 0.1 eV.

The process often used with scientific investigations of moleculereactions to unify the energy of the ions used for the investigation,where they are led through a buffer gas, is not possible here, where anion beam should cause reactions in a considerably evacuated reactionchamber. The application of electrical and magnetical fields alreadyproposed for such cases to mask out a part of the ion beam depending onits energy is too expensive for the commercial use of ion moleculereactions.

The invention is based on the fundamental idea to use just the energydependence of ion molecule reactions, which is the reason why thestabilisation of the ion energy is regarded as being so important, forthe stabilisation. To put this idea into practice quantities had to befound that indeed depend on the energy of the injected ions but not onthe absolute value of the product ion current. This absolute valuenamely depends on different quantities, e.g., on the geometry of thearrangement, on the intensity of the ion current, on the concentrationof the gas to be analysed etc., and therefore does not allow simpleconclusions on the ion energy.

SUMMARY OF THE INVENTION

The solution of the problem put forward according to the inventionprovides, that a calibration gas is filled into the reaction chamber andthat the voltage which accelerates the ions between the ion source andthe reaction chamber is varied until the energy-dependent ratio betweenthe production rates of two reaction products attains a pre-set value oruntil the production rate of a reaction product, the energy-dependenceof said production rate showing an extreme value, reaches said extremevalue.

The two possible realizations of the inventive idea, namely themeasurement of an energy-dependent ratio of product rates or thesearching for the extreme value of a production rate, are described inthe following by an example especially important in practice, where theion source emits Kr⁺ -ions and air is used as calibration gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reaction rate for krypton and oxygen ions versuskinetic energy of the ions.

FIG. 2 shows the reaction rate of krypton ions with nitrogen versuskinetic energy of the ions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The description is carried out by diagrams, where FIG. 1 shows thereaction rate for Kr⁺ and O₂ ⁺ according to T. T. C. Jones, Thesis Univ.Aberystrwyth 1982, and FIG. 2 the reaction rate for the transition ofthe Krypton-ions between different spin states as a result of collisionswith N₂.

The detailed description of the device for carrying out the method canremain undone, as it is already known per se. A schematic description ofan applicable arrangement together with literature references thatexplain particular details can be found in an article by H. Villinger,J. H. Futrell, A. Saxer, R. Richter and W. Lindinger in J. Chem. Phys.80(6), Mar. 15, 1984. The used low energy Krypton-ions can be producedby an ion source which is based on the principle of electron impact.Such ion sources are sold, e.g. by the firm Balzers with the referencenumber BG 528370 T. An octopole-system connected to the ion sourceproduces a high frequency field, that prevents the ion beam fromdiverging. Subsequently the ions enter the reaction chamber, into whichthe gas to be analysed is introduced. A quadrupole mass-spectrometer,that only allows ions of a certain mass to pass on to a standardelectronic ion detection system, serves for the analysation of the beamcoming from the reaction chamber. The density of the gas to be analysedin the reaction region is typically in the order of 10⁻² Torr, whereasthe free path of the gas in the region of ion source and of the massspectrometer should be larger than the dimensions of the device, i.e. avacuum of e.g. 10⁻⁵ Torr is maintained.

To hold the energy of the Krypton-ions at a low reproduceable adjustablevalue between 0.2 and 0.5 eV normal, non-polluted atmospheric air isfilled into the reaction chamber from time to time and the productionrates for O₂ + and CO₂ + are measured. The air used as calibration gascontains

78.084 vol.-% N₂

20.946 vol.-% O₂

0.934 volums-% Argon

0.033 volums-% CO₂

and H₂ O and traces of other gases that can be neglected in thiscontext.

While Kr⁺ does not react with N₂ l or Ar at low energy collisions(KE_(cm) <1.5 eV), it undergoes charge exchange processes both with O₂and CO₂, where the reaction rates in the case of O₂ are dependent bothon the kinetic energy of the collision partners and on the spin state ofthe Kr⁺ ions. This dependence is shown in FIG. 1 where the measuredvalues in black refer to the spin state 3/2 and the data shown in whiterefer to the spin state 1/2. For CO₂ k=6.3×10⁻¹⁰ cm³ sec⁻¹, independentof KE cm and the spin state of the Kr⁺ -ions. In the ion beam comingfrom the ion source Kr⁺ (² P_(1/2)) and Kr⁺ (² P_(3/2)) are present inthe statistical ratio of 1:2 and this ratio does not change in thereaction chamber, although per reactive collision of a Kr⁺ -ion severalcollisions with N.sub. 2 occur. The quench rate, k_(q) for theconversion of the energetically higher Kr⁺ (² P_(1/2)) into Kr⁺ (²P_(3/2)),

    Kr.sup.+ (.sup.2 P.sub.1/2)+N.sub.2.sup.k.sbsp.q Kr.sup.+ (.sup.2 P.sub.3/2)+N.sub.2

is small enough (cf. FIG. 2), so that the ratio Kr⁺ (² P_(1/2)):Kr(²P_(3/2)) in spite of the dominance of N₂ in the calibration gas is notchanged essentially. Thus a ratio of the product ion currents i(O₂⁺):i(CO₂ ⁺) results as ##EQU1## As the production rate of i(CO₂ +) isconstant as mentioned above, the ratio of the product ion currents i(O₂⁺)/i(CO₂ ⁺) changes according to FIG. 1, the left) branch of the valuesshown in white being negligible in practice. This dependence can be usedto detect the energy of the Krypton-ions by measuring the product ionratio at clean dry air, whereupon the extraction voltage or the aperturevoltge of the ion source is varied until the desired value is attained.

If one confines oneself to work at a fixed value of the ion energy of0.3 eV it is sufficient to measure only the product ion current i(O₂ ⁺)and to vary the voltage until said current reaches its minimum.

The invention is not restricted to the embodiment shown. For example itwould be possible, when nitrogen is used as calibrations gas, to utilizein a similar manner the ratio of N₂ ⁺, which is produced mainly at alower energy, and N⁺, which is produced mainly at a higher energy.

I claim:
 1. A process for precisly adjusting the energy of krypton ionscomprising:filling a reaction chamber with a calibration gas containingoxygen and carbon dioxide; emitting krypton ions from an ion source;accelerating the krypton ions by applying a voltage between the ionsource and the reaction chamber; directing the krypton ions into thecalibration gas thereby causing the ion molecule reactions with theoxygen and carbon dioxide; measuring the ratio of oxygen ions and carbondioxide ions by mass spectrometry. adjusting the voltage used toaccelerate the krypton ions until the ratio of oxygen ions and carbondioxide ions attains a preset value.
 2. The process according to claim1, wherein the calibration gas is air.